Several different technologies have been exploited in the past for measuring the strength of high power microwave (HPM) electric fields. D-dot sensors represent the original and most common technology used. More recently, however, field sensors based on electro optic technology have been reported. One type of electro-optic (EO) sensor is based on Mach-Zehnder interferometric technique, and the other type is the EO sensor described in related U.S. patent application Ser. No. 12/205,766, which is based on a polarimetric technique.
Presently D-dot and B-dot sensor probes are used for high power microwave test and evaluation (HPM T&E), where a D-dot sensor is a dipole antenna capable of measuring an electrical field, while a B dot sensor is a coil or loop used for measuring a magnetic field.
The D-dot sensor, used as a dipole antenna, is attached to a copper-jacketed coaxial cable. Because the D-dot sensor itself contains metallic parts and makes use of a coaxial cable, it inevitably perturbs the field that it measures. Thus, because of this perturbation, the D-dot sensor is unable to measure the true field accurately.
Limitations and disadvantages of the conventional D-dot and B-dot sensors include: (1) unacceptably large field perturbations, (2) narrow bandwidth, and (3) bulky physical size. Both the D-dot and the B-dot sensors are composed of metallic parts, and hence they perturb fields that they measure, based on the metallic composition of the sensors. The large field perturbations render these sensors unable to accurately measure true electrical and magnetic fields.
D-dot sensors have relatively narrow frequency bandwidths, typically less than 1 GHz; hence they are unsuitable for many wideband or ultra-wideband HPM field tests. In addition, conventional electric-field sensors (such as D-dot sensors) are relatively large in size; hence they are unable to measure the smaller details in an electric field pattern, or to detect fields in enclosed spaces containing electronic circuits. In fact, early attempts to characterize the microwave radiation inside a cavity using a D-dot sensor completely failed because of the above limitations and design problems.
Electro-optic (EO) sensors, used for the nonperturbative detection of electric fields, are typically configured to modulate an optical probe beam along a crystal direction in which the EO tensor coefficient is largest. However in fiber optic EO sensors, such configurations can be strongly limited by phase noise and cumbersome compensation optics. A greater signal to noise ratio can be achieved by modulating along a crystal direction of low static birefringence, even when the active EO tensor coefficient is several times smaller.
The use of electro-optic (EO) crystals in electric field measurement applications has gained considerable momentum over recent years. This is due to their compact size (<1 cm3), large dynamic range (˜1 MV/m), large intrinsic bandwidths (DC-THz), and ability to simultaneously measure both field amplitude and phase. EO field sensors measure electric fields through a phase modulation induced in a probe beam passing through a nonlinear crystal. The phase modulation is due to a distortion of the optical indicatrix (linear EO effect), and is converted to a beam intensity modulation using an analyzer (polarizer). The modulation depth is directly proportional to the applied field, allowing the field to be directly measured (within a proportionality factor) using a photodetector. Unlike conventional metallic field probes, EO sensors are all-dielectric and can be utilized in large electric fields or high power microwave (HPM) fields. In spite of these advantages, metallic probes are still predominantly used, while EO sensors are usually confined to more research-oriented laboratory applications. One reason for this is that EO sensors are susceptible to phase noise and sensitivity losses that are greatly compounded when extended lengths of optical fiber (e.g., 50 meters) are used. The use of long optical fibers is often unavoidable in field testing, especially at HPM test sites where the readout instruments must be remotely isolated to avoid interference from stray or reflected fields.
Noise level generally increases linearly with the length of the fiber. Investigations have revealed that the noise originates in the phase of the optical signal. This was determined by examining the noise in orthogonally polarized components of the probe beam using a Wollaston prism in place of the polarizer. These noise components exhibited equal and opposite amplitudes, indicating their origin in the phase of the optical signal. Because of the polarization dependence of the noise, subtracting the two signals (a common noise cancellation technique in commercial balanced photoreceivers) can not be effectively utilized.
In attempts to solve some of the above mentioned problems, an electro optic field sensor based on the Mach Zehnder interferometer technique has been invented. However, the Mach Zehnder electro optic sensor is incapable of measuring the phase or the waveform of the electrical and/or magnetic fields. The Mach-Zehnder sensor is also made of dielectric materials and optical fibers, but in contrast to the polarimetric design, the Mach-Zehnder sensor is also incompatible with long optical fibers.
Polarizers are traditionally used to analyze phase modulated probe beams, and convert them into intensity modulated optical signals.
Optical instabilities and/or small variations in the optical path length within a modulating medium (i.e., electro optic crystals and electro optic polymers) can produce amplitude fluctuations and phase fluctuations of an output optical signal. Constant manual adjustments of the traditional polarizer rotation angle can improve the stability to a certain degree; however, such manual adjustments are cumbersome and can not always recover the signal loss. No device hitherto exists to eliminate these instabilities and keep the device operating at its optimal responsivity.
Therefore, the need exists for devices and systems capable of measuring the phase and/or the waveform of electrical and/or magnetic fields, while reducing large field perturbations resulting from the metallic composition of measurement sensor probes.
Also, the need exists for less bulky physical sized devices and systems directed to measuring the phase and/or the waveform of electrical and/or magnetic fields, where these systems include wideband high power microwave test and evaluation capabilities.
Finally, the need exists for an EO sensor design suitable for HPM T&E, which does not generate polarization noise when long optical fibers are attached to the sensor.